Recirculation is a technology which can be employed basically for the most diverse possible purposes in gas turbines. Thus, for example, for controlling the emissions, reducing the exhaust gas volume for carbon dioxide separation, etc. In recirculation of exhaust gases in a gas turbine, a substantial fraction of the exhaust gas is branched off from the overall exhaust gas substream and is normally delivered again, after cooling and purification, to the mass entry stream of the turbine or to the turbine compressor. The exhaust gas composition differs considerably from the composition of fresh ambient air. Conventionally, the recirculated exhaust gas substream is mixed with fresh air from the surroundings and this mixture is subsequently delivered to the compressor.
Advantageously, by exhaust gas recirculation, the carbon dioxide partial pressure in the exhaust gases can be increased, in order to reduce the power losses and efficiency losses of power plants having carbon dioxide separation. Further, exhaust gas recirculation has been proposed for the purpose of reducing the oxygen content in the intake gases of gas turbines, in order thereby to reduce the NOx emissions.
For exhaust gas recirculation, for example, U.S. Pat. No. 7,536,252 B1 describes a method for controlling an exhaust gas recirculation stream of a turbo machine, which is returned to the inlet of the turbo machine via an exhaust gas recirculation system. In this method, a desired exhaust gas recirculation fraction, which contains the fraction of the exhaust gas substream in the entry stream of the turbo machine, is determined, and the actual value is set to the desired value.
EP2248999 discloses a power plant with exhaust gas recirculation and a method for operating a power plant of this type, in that the recirculation rate and the temperature to which the recirculated exhaust gases are recooled are controlleded as a function of the load.
In order to make recirculation of exhaust gases possible, it is proposed in the prior art to provide blowers in order to overcome the pressure losses in the recirculation line, in the recooler for the recirculated exhaust gases, etc. Alternatively, there are versions in which the recirculation line, the cooler for the recirculated exhaust gases, etc. are designed to be so large that the flow velocities and therefore the pressure losses are so low that overpressure in the exhaust gas lines is sufficient to return the exhaust gases. Both solutions are large and costly and require additional space in a power plant. In particular, the use of a blower is further detrimental to the power and efficiency of the power plant.